Effect of Wet and Dry Finishing and Polishing Technique on Microhardness and Flexural Strength of Nanocomposite Resins

Objectives This in vitro study was aimed to assess the effect of wet and dry finishing and polishing techniques on the flexural strength and microhardness of different commercial nanoparticle contained composite resins. Methods and Materials The samples were made of Z250 (microhybrid), Z350 XT (nanofilled), and Z550 (nanohybrid) resin composites. Each group was subdivided into 2 subgroups according to polishing protocols. Subgroup 1 for each composite underwent wet polishing, and subgroup 2 was subject to dry polishing technique. Flexural strength and microhardness of the samples were measured at two different times of polishing (T0 and T24). The flexural strength test and microhardness test were measured by a 3-point bending test using a universal testing machine, and a Vickers machine, respectively. Data were analyzed by Kolmogorov–Smirnov, two-way ANOVA, and Tukey HSD tests. Results ANOVA showed that the type of composite has a significant effect on flexural strength. Two-way ANOVA showed that, at T0, flexural strength of all composites in the dry technique was higher than in the wet technique (p = 0.019). At T24, Z350 XT had the lowest, and Z250 had the highest flexural strength in both techniques. The time and technique of polishing were also significantly effective on hardness. At T0, hardness was higher in the wet compared to the dry method (p = 0.008). Tukey test showed that, at T24, the hardness of Z350 XT was significantly higher than the other materials in both techniques. Conclusion Immediate wet finishing and polishing presented lower flexural strength. Delayed dry/wet finishing and polishing significantly enhanced the hardness of the samples.


Introduction
Nowadays, not only patients but clinicians also prefer composite resins as a tooth-colored material because of their good aesthetic quality and physical and mechanical properties [1]. A large part of the weight and volume of composite resins consists of fllers [2]. Fillers are responsible for strengthening the resin matrix, creating translucency, and abating the shrinkage caused by polymerization. More fller particles facilitate the clinical application of composite resins. Also, the particle size of fllers has a distinct efect on the mechanical properties of particulate-polymer composites. Fillers with smaller particle sizes have increased particle surface area, which results in a high surface energy at the fller-matrix interface [2]. Terefore, the smaller the size of the fller particles, the more they can coalesce into the structure of the composite material [2]. Following progression in nanoparticle development, it has been postulated that nanocontained resin composites possess favorable mechanical properties. Adding nanoparticle fllers to the resin matrix of dental composites improves aesthetic, optical, mechanical properties, wear resistance, and gloss retention and reduces polymerization shrinkage [2].
Te uneven or rough surface of a restoration due to improper fnishing and polishing leads to staining, accumulation of plaque, recurrent carries, wear, and reduced longevity and durability of the restoration [3]. Tus, fnishing and polishing procedures are required for dental restorations to meet the requisite aesthetic, function, and periodontal health properties of dental restorations [4,5].
Diferent methods are available for fnishing and polishing. Te composition of the composite, the presence of air incorporated in the composite, and the instruments used are the most important factors which can infuence the surface glossiness and smoothness [6].
Te best time to fnish and polish composite restorations is a controversial matter. Although composite manufacturers recommend that the procedure must be better carried out immediately after restoration, research shows that it must be postponed to prevent the side efects of heat generation including smearing the resin matrix and creating local hotspots before the fnal polymerization of the composite [7]. But the fact is that delayed fnishing and polishing can also cause disadvantages such as lower microhardness. Tis may be related to the loss of surface properties after polymerization and the stress produced during the delayed polishing procedure. In this way, delayed polishing procedures can compromise the marginal sealing obtained with the hygroscopic expansion of the composite and adhesive system, resulting in an increase in microleakage due to the stresses and the heat generated by the procedures. However, in the immediate polishing technique, hygroscopic expansion after several days can compensate for the damage caused by the procedures and improve the marginal sealing [8].
Besides the time of polishing, the method of polishing can also be considered of great importance. Polishing can be carried out by dry or wet methods. Te dry method can cause a better view and administration of the work area, but it generates a lot of heat, which can afect the restoration properties. In the wet technique, a water coolant is used to decrease the temperature in order to prevent the damages caused by heat [9].
Tere is no consensus on which condition provides the best surfaces for nanoparticle contained resin composites. Te objective of the present study was to evaluate the efect of immediate or delayed dry and wet fnishing/polishing on the surface hardness and fexural strength of composite resins.

Specimens Preparation for Flexural Test.
Te samples for this test were prepared as per ISO 4049 standards. 20 samples were made from each material according to a similar previous study [5]. A stainless steel mold of 25 × 2 × 2 mm was used. Te mold was placed on a glass slab. Te composite was packed into the mold until it was full. Ten, a transparent mylar strip was placed on the composite and a glass slab over the mylar strip. In order to eliminate the excess composite, the glass lam was pressed slightly. Ten, using a halogen light cure device (Optilux 501, Kerr Manufacturing Inc, Orange, CA, USA), each specimen was cured in 3 areas with a light intensity of 600 mW/cm 2 for 60 seconds according to the manufacturer's instructions. First, the central area and then the two sides were cured using an overlapping manner to make sure that all areas have been covered. Ten, the specimens were taken out of the mold and their dimensions were measured using a digital caliper (Digimatic caliper, Mitutoyo Corp., Tokyo, Japan).

Specimens Preparation for Hardness
Test. Twenty disk shape samples of each composite were made with a 6 mm diameter and 2 mm height. Te method for preparing samples was similar to that of the fexural strength test except for mold shape. Curing was carried out using a halogen light cure device (Optilux 501, Kerr Manufacturing Inc, Orange, CA, USA) with an intensity of 600 mW/cm 2 for 20 seconds according to the manufacturer's instructions. Te tip of the device was in complete contact with the covering glass tip, and perpendicular to the center of the sample. All fnishing and polishing procedures were performed by the same operator, who was blinded to the group allocation of samples. Finishing and polishing were carried out using OptiDisc Kit (KerrHawe SA, Switzerland). Te disks were used in the order of medium, fne, and extrafne grit sizes by a low-speed (5000 rpm) hand piece (Ti-Max Electric hand piece; NSK, Tokyo, Japan).
For dry samples each specimen was polished for 15 seconds by each disk, using a rotating movement with slight constant pressure. For wet samples, the same procedure was carried out, but it was carried out using a waterjet with a fow rate of 20 cc/min as coolant. For dry groups, between changing the disks, the sample was cleaned with a soft tissue paper to remove the debris, while for the wet samples, they were washed with water for 5 seconds between the disks. For immediate samples, whether dry or wet, fnishing and polishing were carried out immediately after curing. For 24 hour groups, after curing, samples were immersed in deionized water, and incubated in 37°C (PECO PL-455, Pooya Electronic Companic CO, Iran) for 24 hours, before fnishing and polishing.

Assessment
Procedure. 3 point bending test was carried out using a universal testing machine (Zwick/Roell Z020, Ulm, Germany) for assessing fexural strength. According to ISO 4049 standard, the amount of loading force was 50 ± 16 N/min and the speed was 0.5 mm/min. Te maximum amount of load that the sample was able to withstand before failure, was calculated as the fexural strength of that sample. FS was calculated from the following equation: where F is the maximum load exerted on the specimen in newton (N), L is the distance between supports in millimeters, b is the specimen width in millimeter, and h is the height of the specimen in millimeters. Te obtained data were reported in MPa. A microhardness test was carried out by a Vickers Machine (Zwick/Roell Indentec, Ulm, Germany). Te measuring precision of the device was 0.025 microns. At frst, surface of the sample was observed with 125x magnifcation. A smooth area was chosen and an indentation was made on it with a 200-gram load for 15 seconds. Ten, the indentation was measured with 125x magnifcation, and microhardness was calculated. Te test was carried out 3 times for each sample, with a 1.5 mm distance between indentations. Te average of the three measurements was calculated as the microhardness of the sample.
Data were analyzed by Kolmogorov-Smirnov, two-way ANOVA, and Tukey HSD tests using SPSS software (IBM SPSS Statistics version 27, Armonk, NY, USA). Te level of signifcance was p < 0.05 for all tests.

Results
Tables 2-5 and Figures 1 and 2 show the fexural strength and hardness of composite resins subjected to diferent fnishing and polishing protocols.
Te results showed that in immediate fnishing and polishing, the type of composite did not have any efect on fexural strength (p � 0.025), but the interaction efect of fnishing and polishing protocols on fexural strength values was signifcant (p � 0.019) and all the dry samples showed higher fexural strength than the wet samples. In 24 hour groups, the composite type was the determining factor (p � 0.026). Te Z250 samples showed the highest fexural strength in both techniques while the Z350 XT samples showed the least.
Microhardness results showed that in immediate fnishing and polishing, both the composite type and the technique were efective factors, and the microhardness of all of the wet samples was higher than the dry ones (p � 0.008).
Tukey's test showed that in immediate fnishing and polishing in both techniques, Z350 XT samples had the lowest microhardness values. In 24 hour groups, Z250 and Z550 samples, showed higher microhardness in the wet technique compared to the dry technique.
In 24 hour groups in the dry technique, the type of composite was an afective factor; Z350 XT samples had the highest microhardness values compared to Z250 and Z550 samples (p < 0.001), and Z550 samples were harder than Z250 (p � 0.044).

Discussion
Surface smoothness is a property resulting from the interaction of many factors. Tese factors are divided into intrinsic and extrinsic factors. Intrinsic factors such as fller type, shape, size, and distribution, the type of resin matrix, the degree of fnal cure achieved, and the bond efciency at the fller/matrix interface can infuence the surface roughness or smoothness. Extrinsic factors are related to the type of polishing system used, such as the fexibility of the material in which the abrasives are incorporated, the hardness of the abrasives, the geometry of the instruments, and the used method [6,[10][11][12][13].
Delivering a realistic restoration which mimics natural tooth structure is one of the dentist's responsibilities. Finishing and polishing procedures can afect both the aesthetics as well as the mechanical properties of the restoration [14]. Finishing and polishing can be carried out by two methods: dry and wet. Each of these techniques has pros and cons which can afect the fnal properties of the restoration [15,16].
According to the results of this study, at T 0, type of material had no efect on fexural strength, while Ramirez and Kaplan [17] and Junior et al. [18] came to the opposite conclusion. Te reason may be related to the fact that they tested the fexural strength after 24 hours and after 7 days of keeping the samples in water, respectively. During this period, probably dark polymerization has taken place in the samples [9], which allows the material to show its fnal properties. We have carried out the test immediately after curing. At T 24 , we reached similar results as theirs, and our results showed a signifcant relationship between the type of material and fexural strength. Junior et al. [18] concluded that the fller content of composite resin materials afects their fexural strength. Te factors they mentioned in this regard were weight percentage, shape, type, and sealant coverage of fller particles, and also the resin matrix composition [18]. In this study, our result showed that Z350XT as a nanoflled composite demonstrated the lowest fexural strength and highest hardness at T 24 under dry conditions. Composite strength depends on the load transfer between fller and matrix. Because of forming large nanofller aggregate in this nanoflled composite, the interfacial area between nanofller layers and polymers are reduced and this composite cannot properly transfer the load between these 2 phases, which leads to less fexural strength [19].
Regarding the independence of fexural strength from the technique at T 24 , the reason may be that after dark polymerization had taken place, and cross-linking was completed, generated heat from polishing was not able to change the mechanical characteristics [20]. Also, after 24 hours, due to water absorption and the plasticizer efect of water, the strength of the samples may have altered [4].
According to the fndings of the present study, at T 0 , wet and dry techniques had a signifcant efect on fexural strength, but at T 24 , they were irrelevant. At T 0 , dry samples showed higher values for fexural strength. Nasoohi et al. [21] also concluded that temperature elevation caused by dry polishing increases the amount of cross-linking and thus improves the mechanical properties.
In two of the composites, Z550 and Z250, there is actually a reduction in the surface hardness at 24 hr in the dry group compared to the wet group. Tese results are in accordance with the study conducted by Kaminedi et al. In his study, the hybrid resin composites (like Filtek ™ Z250 and    to the maturation of the resin matrix by the heat generated with no cooling system. However, in the hybrid composites, the particle size is larger, leaving the surface rough due to the plucking out of fller particles after wearing out of the resin matrix during polishing and the produced heat [9].   At T 0 , microhardness results showed a signifcant relationship with the type of material. Similar results to the results of the present study were observed in various studies [9,13,20]. In the study of Nasoohi et al. [20], both technique (wet/dry) and type of material could afect the microhardness. At T 24 , the technique was signifcantly relevant to microhardness. Nasoohi et al. [21] and Kaminedi et al. [9] have found similar results in their research. But in their studies, dry samples showed higher values, while our results showed a diferent outcome. Tis controversy may be due to the use of diferent polishing tools. Margio et al. [15] showed that tool characteristics can afect the mechanical properties of composites. Our polishing kit (OptiDisc, kerr, Middleton, WI, USA) may have generated less heat and thus glass transition had not taken place. Or, maybe the kit has generated a such large amount of heat that has burnt the polymer and thus caused a decreasing in the hardness. In addition, other variables can infuence the hardness and fexural strength, such as curing technique and surface treatment [20,22]. All these factors should be considered in future studies.
Another important issue is the efect of fnishing and polishing time on the microleakage of composite restorations. Studies showed that the fnishing and polishing time was efective on the mean microleakage in the enamel margin of composite restorations. Time of fnishing and polishing had no efect on microleakage in dentin margins of restorations. Immediate fnishing of the repaired restorations negatively afects the sealing at the repair interface, while 20-minute and 24-hour delayed fnishing had no adverse efect on the interface sealing. Te best time for fnishing and polishing was 24 h after the restoration [23,24].
Te limitation of the present study involves the in-vitro methodology that can lead to a diferent result in clinical situations as there are other intervening factors. Also, in this study other variables such as curing technique, diferent polishing systems, and other mechanical factors are not considered.
Considering the limitations of this in vitro study, we can conclude that the mechanical properties of composite resins are multifactorial issues and it may not be possible to assess the efect of a single factor independently.

Conclusion
Immediate wet fnishing and polishing presented lower fexural strength. Delayed dry/wet fnishing and polishing signifcantly enhanced the hardness of the samples.

Data Availability
Te data used to support the study are included in the paper.

Disclosure
Tis research was performed as part of the employment of all the authors.

Conflicts of Interest
Te authors declare that they have no conficts of interest.